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Homework Help: Virtual Work Method on a Truss

  1. Mar 18, 2014 #1
    1. The problem statement, all variables and given/known data
    I want to calculate the horizontal deflections at joints A and F (using Virtual Work method taught in Structural Engineering). Cross-Sectional Area (A) and E values are provided in the problem, and the attachment has all relevant dimensions and applied loads.

    2. Relevant equations

    Deflection is 1[k](Δ)=∑(Virtual Force/AE)

    3. The attempt at a solution

    So far in class, Virtual Work problems have involved using the Unit Load Method (a hypothetical 1 kip load that can be used to calculate deflection from its effect on a given structure).

    The general method (as I understand it, in a truss structure) is this:

    1. Solve all member forces.

    2. Organize all information (member length in inches, member force, cross-sec area, and MEMBER FORCE FROM UNIT LOADING into a table.

    3. Summation of Virtual Forces from table plugged into Deflection Equation from Virtual Work.

    Step 1. I've already done myself (joint method), and Step 3. is a matter of plugging in known data from Step 2. No questions there.

    Step 2. is where my question is.

    This particular question wants horizontal deflections at A and also F. What I think I should do is introduce a horizontal 1 kip load at each of the respective joints, and repeat the joint analysis to reflect this change from the unit load. But in what direction (sign convention) should they be introduced? Should I introduce a horizontal 1 kip load at EACH joint, or only the requested joints?

    Some problems I've seen introduce the unit load at only the requested joint, and recalculate all other member forces to adjust for that change (which is what I'm leaning towards doing), but others introduce the unit load immediately, without taking initial member forces. This is confusing, and so I'm asking for a clarification in procedure.

    No numbers are necessary. Thanks everyone!

    Attached Files:

    • Q4P1.png
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    Last edited: Mar 18, 2014
  2. jcsd
  3. Mar 19, 2014 #2
    If you want the deflection at a particular point, apply the unit load there, and only there. The direction doesn't really matter, but if the answer is negative, that means it was the other way. Still correct though. So you have the frame analyzed with the given loads, and secondly, you have the frame analyzed with just the unit load. Do you know how to combine these two results to give you the deflection you want?
  4. Mar 19, 2014 #3
    pongo38, thanks for your reply!

    Regarding the two results for deflection of A and F, aren't they separate deflections? Like, for A I apply a unit load at A horizontally, and calculate changes in member forces, and multiply in the PL/AE equation eventually? Then do the same thing for F, and have two separate deflections?

    I thought that for however many deflections you wanted, you had to have that many virtual structures? Are you saying that I apply a unit load to the truss at A and F at the same time?
  5. Mar 19, 2014 #4
    Not at the same time. Two separate processes, one for each deflection required. However, there is a matrix method that does it all in one go. But that is not the method you are pursuing here.
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